[998-30-1] · C6H16O3Si · Triethoxysilane · (MW 164.31)
(useful reagent for the hydrosilylation of carbon-carbon multiple bonds; reducing agent for carbonyl groups)
Physical Data: bp 134-135 °C; d 0.89 g cm-3.
Solubility: sol diethyl ether, THF, alkanes, aromatic and chlorinated solvents.
Form Supplied in: colorless liquid; widely available.
Handling, Storage, and Precautions: is moisture sensitive and should be handled in a well-ventilated hood. Contact with the eyes and skin should be avoided.
HSi(OEt)3 is an efficient reagent for the hydrosilylation of alkenes, alkynes, and conjugated dienes (eq 1).1 The reaction is promoted by a variety of transition metal catalysts, including platinum, ruthenium, and rhodium. Chloroplatinic acid hexahydrate, H2PtCl6.6H2O (see Hydrogen Hexachloroplatinate(IV)), is by far the most widely used catalyst and has been shown to be very efficient, particularly in the hydrosilylation of alkenic substrates.2
The reaction is especially valuable for the introduction of functionalized groups at silicon,3 as this cannot be achieved by simple organometallic procedures (eq 1).4 Furthermore, the reaction may be quite regioselective and stereoselective, as in the case of the hydrosilylation of ocimene (eq 2).5
A general and powerful reduction method of carbonyl compounds consists of using HSi(OEt)3 activated by Potassium Fluoride or Cesium Fluoride.7 The reaction can proceed without solvent (eq 3)7a and is accelerated by polar solvents, e.g. DMSO or DMF.8 The reduction of aldehydes and ketones is rapid at room temperature; also, esters are smoothly converted to alcohols (eq 4).7b
The reaction is highly chemoselective: aldehydes and ketones possessing functional groups, such as carbon-carbon double bonds, or bromo, nitro, amide, and ester groups, are reduced selectively to the corresponding alcohols (eqs 5 and 6).7a,9
Alternatively, solutions of HSi(OEt)3 with lithium ethoxide or pinacolate may be used to convert aldehydes and ketones to alcohols.10 Enantioselective reduction of prochiral ketones is achieved by use of a mixture of HSi(OEt)3 and the dilithium salt of a chiral diol or amino alcohol.11
The reaction between carbonyl compounds and HSi(OEt)3 is efficiently catalyzed by inorganic solid bases such as hydroxyapatite, Ca10(PO4)6(OH)2, at temperatures ranging from 25 to 90 °C.12 Enones afford the corresponding 1,2-addition products.
HSi(OEt)3 is easily converted into a more reactive form, namely a pentacoordinate hydridosilicate (eq 7).13a
K[HSi(OEt)4] reduces aldehydes, ketones, and esters in the absence of added catalyst.13 Treatment of nonenolizable amides yields the corresponding aldehydes. Reaction with 1 equiv of isocyanate in diethyl ether or THF leads quantitatively to the potassium imidate, which can be quenched in situ (eq 8).13b Furthermore, use of an excess of aryl isocyanate yields to the corresponding isocyanurate.13b
HSi(OEt)3 is used stoichiometrically in conjunction with a titanium-based catalytic system, conveniently prepared by the reaction of 2 equiv of n-Butyllithium with Dichlorobis(cyclopentadienyl)titanium (eq 9).14a
The combination of a catalytic amount of Titanium Tetraisopropoxide, an inexpensive and air-stable liquid, and HSi(OEt)3 also generates an effective and mild system for the conversion of esters into primary alcohols.14b
Selective reduction of a great variety of esters has been achieved with these methods. The procedures represent safer and convenient alternatives to those employing reducing agents such as Diisobutylaluminum Hydride and Lithium Aluminum Hydride.
Robert J. P. Corriu & Christian Guérin
Université Montpellier II, France